The invention relates to multimode multi-track optical reading and recording using multimode laser diodes.
Semiconductor laser diodes are available as single mode or multimode diodes. The radiation emissions of single mode laser diodes are effectively modelled as point sources and are diffraction limited in their divergence on all axes. In contrast, multimode diodes typically have laser junctions which emit radiation along stripes having an elongated axis and a short axis; for this reason multimode diodes are often referred to as xe2x80x9cstripexe2x80x9d type laser diodes. Multimode laser diodes are diffraction limited in the direction perpendicular to the junction (their short axis), but have non-diffraction limited divergence in the direction parallel to the laser junction (their elongated axis).
The region through which radiation emitted from a diode""s laser junction is permitted to escape into the environment surrounding the diode is referred to as the xe2x80x9cemitting aperturexe2x80x9d of the diode. The emitting aperture of a multimode diode is generally elongated and can comprise a single or continuous stripe, a collection of short stripes or even a collection of single mode laser junctions electrically connected in parallel. In this document, the phrases xe2x80x9cmultimode diodexe2x80x9d and xe2x80x9cmultimode laser diodexe2x80x9d should be understood to incorporate each of these different diode constructions. In addition, laser diodes can emit radiation of various different frequencies and any reference to xe2x80x9clightxe2x80x9d in this document should be understood to incorporate any radiation frequency. Furthermore, reference is made throughout this document to the xe2x80x9cshort axisxe2x80x9d and the xe2x80x9celongated axisxe2x80x9d. These axes relate to the stripe shape of the emitting aperture of a multimode diode, but are also used as convenient references to directions in space (e.g. a direction may be described as being parallel to the elongated axis).
For recording applications, the principal advantage of using multimode laser diodes is that the radiation emitted from multimode diodes can be of substantially higher power than that emitted from single mode diodes. Obviously, higher power is a desirable quality for a recording operation, where heat or optical power alter the physical characteristics of the recording media. Despite this advantageous characteristic, multimode laser diodes are often problematic to use for image recording, because of difficulty associated with their non-uniform near-field power distribution. Not only is the near-field power distribution of a multimode diode non-uniform, but it typically changes with the age and usage of the diode. In an optical recording device, this non-uniformity of the near-field power distribution leads to an unacceptable phenomenon on the recording media known as xe2x80x9cbandingxe2x80x9d, where the recorded image may be significantly degraded. Because the non-uniform power distribution of multimode diodes may lead to data loss or corruption, most optical recording devices employ single mode laser diodes, despite their relatively low power.
Accordingly, there is a need to improve the performance of multimode laser optical recording to overcome difficulties associated with the non-uniformity in the near-field power distribution of multimode laser diodes.
Various attempts have been made in the prior art to address the non-uniformity of the near-field power distribution of multimode laser diodes. One solution to this problem is to combine several diode emitters onto a single focal area. In this manner, the overlapping or combining radiation from several diodes can be used to effectively xe2x80x9caverage outxe2x80x9d the non-uniformities of any single diode. This xe2x80x9cemitter combinationxe2x80x9d technique is exemplified by U.S. Pat. Nos. 5,517,359, 5,923,475, and 6,064,528, all of which employ several laser diodes to illuminate the entrance pupil of a light valve (also known as a xe2x80x9cspatial light modulatorxe2x80x9d). U.S. Pat. No. 5,793,783 employs a similar emitter combination technique, using several diodes to directly illuminate a single spot on a recording surface.
The principal drawback with the emitter combination technique is that it is inefficient in terms of both energy and space. The energy inefficiency results from the overlap of radiation from several diodes onto a single focal area. Today""s commercially available multimode laser diodes produce sufficient power to individually image many types of media (i.e. without combining radiation from several diodes). While, the redundancy of the emitter combination technique does achieve a inure uniform power distribution, it is inefficient, because an individual diode can supply sufficient energy to image the recording media, and any excess energy contributed by the overlapping radiation of several diodes is wasted. The spatial inefficiency results from the need to have several distinct diodes for each focal area.
A second technique demonstrated by the prior art to overcome the non-uniformity of the near-field distribution of multimode laser diodes is to simply image the diode""s far-field power distribution rather than its near-field power distribution. This technique is exemplified in U.S. Pat. Nos. 5,745,153 and 5,995,475.
A drawback with imaging the far-field distribution of a multimode laser diode is that it requires a relatively large amount of space. For use in high resolution imaging applications, a relatively large far field pattern must be reduced to a small spot at the recording surface. To image the far-field power distribution of a laser diode and have a large amount of optical reduction requires a relatively long optical path length.
An additional drawback of imaging the far-field distribution is that it adds optical aberrations in both the short and elongated axes of the laser diode image. As mentioned above, the shape of the multimode laser diode junction causes the divergence of the diode radiation to be diffraction limited in a direction parallel to the short axis of the diode emitting aperture. Therefore, additional aberrations in the direction of the short axis of the diode emitting aperture are undesirable.
In accordance with the present invention, a monolithic array of individually addressable multimode laser diodes is employed to image the surface of a radiation sensitive material. The emitting apertures of the diodes each have a short axis and an elongated axis. Input information is received by each of the diodes and is incorporated into a radiation pattern emitted by that diode.
The radiation patterns of each diode are directed toward the radiation sensitive surface by an anamorphic optical subsystem. An anamorphic optical system has different magnification properties on its different axes. For example, a cylindrical lens has no magnification in the direction parallel to the axis of the cylinder. The anamorphic optical subsystem introduces an astigmatism into the radiation patterns of each of the diodes. At the surface of the radiation sensitive material, the astigmatism introduced by the anamorphic optical subsystem causes the images of the emitting apertures of the diodes to be more focused on their short axes than on their elongated axes. In this mariner, the information contained in the radiation patterns can be recorded onto the radiation sensitive material without revealing their near field non-uniformities.
Advantageously, the short axis of the radiation patterns of the diodes may be substantially focused.
Preferably, the images of the emitting apertures of the diodes may be blurred on their elongated axes, such that, at the surface of the radiation sensitive material, their power distribution in their elongated axes is substantially uniform.
Preferably, the elongated axes of the images of the emitting apertures of the diodes on the radiation sensitive material may be between 1 and 5 times the size that they would have been bad they been focused at the surface of the radiation sensitive material.
Advantageously, the optical subsystem may employ at least one cylindrical lens.
Advantageously, the optical recording system may be used to simultaneously record a plurality of data channels on die surface of the radiation sensitive material.
Another aspect of the present invention involves a method of optically recording information onto the surface of a radiation sensitive material using a monolithic array of individually addressable multimode laser diodes. An emitting aperture of each of the diodes has a short axis and an elongated axis.
The first step of the invention involves incorporating input information into the radiation patterns emitted by each of the diodes and optically directing those radiation patterns toward the radiation sensitive material. The next step involves introducing an astigmatism into the radiation patterns prior to their reaching the surface of the radiation sensitive material, such that, at the surface of the radiation sensitive material, the images of the emitting apertures of the diodes are more focused on their short axes than on their elongated axes. In this manner, the information contained in the radiation patterns can be recorded onto the radiation sensitive material without revealing their near field non-uniformities.
Advantageously, the short axis of the radiation patterns of the diodes may be substantially focused.
Another aspect of the present invention involves a method of optically recording information onto the surface of a radiation sensitive material using a monolithic array of individually addressable multimode laser diodes. An emitting aperture of each of the diodes has a short axis and an elongated axis.
The first step of the invention involves incorporating input information into the radiation patterns emitted by each of the diodes and optically directing those radiation patterns toward the radiation sensitive material. The next step involves focusing an image of each of the emitting apertures on its short axis on the surface of the radiation sensitive material, while simultaneously blurring an image of each of the emitting apertures on its elongated axis at the surface of the radiation sensitive material. In this manner, the information contained in the radiation patterns can be recorded onto the radiation sensitive material without revealing their near field non-uniformities.
These and other objects of the present invention will be better understood from the following more detailed description along with the drawings and the accompanying claims.
FIG. 1 depicts a preferred embodiment of the invention employed in a multimode multi-track optical recording system.
FIG. 1A is a schematic representation of the emitting aperture of a multimode diode.
FIG. 2-A depicts a typical non-uniform near-field power distribution profile of the elongated axis of a multimode diode.
FIG. 2-B depicts a substantially more uniform power distribution profile of the elongated axis of a multimode diode in accordance with the present invention.